The fluvial megafan of Abarkoh Basin (Central Iran): an example of flash-flood sedimentation in arid lands
Nasser Arzani, 2005. "The fluvial megafan of Abarkoh Basin (Central Iran): an example of flash-flood sedimentation in arid lands", Alluvial Fans: Geomorphology, Sedimentology, Dynamics, A. M. Harvey, A. E. Mather, M. Stokes
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The Abarkoh Basin is situated in central part of the NW–SE-trending Gavkhoni–Sirjan depression of Central Iran. The studied fluvial megafan represents a Quaternary fan, which is located in the western part of this intermontane, extensional basin. This fan covers an area of 940 km2 and with a gentle slope (about 0.5°) over a 45 km-length terminates in a playa lake in the centre of the Abarkoh Basin. Its catchment is the Abadeh Basin (>2000 km2), which is a NW–SE-trending intermontane basin and mainly floored with the Quaternary alluvial-fan sediments. The studied megafan was built by stream-dominated processes, as revealed by the presence of thick-bedded, clast-supported gravels–sandy gravels (gravelly–sandy conglomerates) in the proximal-fan areas that grade into thick marls with sinuous, lenticular channel-filled, grain-supported, imbricated, pebbly conglomerates in the proximal–medial fan regions. Thick calcareous marls and lenticular, single channel-fill sandstone–conglomerate deposits characterize the distal part of the fan and pass laterally into sediments of the playa lake fringe. The aim of this paper is to present an example of a fluvial megafan in an arid–semi-arid setting, to highlight the flash-flood, sheetflood–channelized sedimentation in megafans, and to stress their importance in water resources in arid lands. Fluvial fans are frequently identified in the rock record. The fluvial megafan of the Abarkoh Basin represents one of only a few described, large-scale, stream-dominated, Quaternary megafans in arid–semi-arid settings.
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Alluvial fans are important sedimentary environments. They trap sediment delivered from mountain source areas, and exert an important control on the delivery of sediment to downstream environments, to axial drainages and to sedimentary basins. They preserve a sensitive record of environmental change within the mountain source areas. Alluvial fan geomorphology and sedimentology reflect not only drainage basin size and geology, but change in response to tectonic, climatic and base-level controls. One of the challenges facing alluvial fan research is to resolve how these gross controls are reflected in alluvial fan dynamics and to apply the results of studies of modern fan processes and Quaternary fans to the understanding of sedimentary sequences in the rock record. This volume includes papers based on up-to-date research, and focuses on three themes: alluvial fan processes, dynamics of Quaternary alluvial fans and fan sedimentary sequences. Linking the papers is an emphasis on the controls of fan geomorphology, sedimentology and dynamics. This provides a basis for integration between geomorphological and sedimentological approaches, and an understanding how fluvial systems respond to tectonic, climatic and base-level changes.